Heretofore, heat-shrinkable polyester films (hereinafter, occasionally referred to only as the film) have been widely used as shrink (accumulative) packaging products, shrink labels, cap seals, etc., for a variety of containers such as polyethyleneterephthalate (PET) containers, polyethylene containers, glass containers, utilizing the heat shrinkable property thereof.
For production of labels or the like, the following procedures are usually employed. Namely, a raw polymer in a molten state is continuously extruded into an undrawn film. Subsequently, the undrawn film is drawn and wound to give a heat-shrinkable polyester film roll. The film in the roll is then unwound and slit into a film with a desired width, which is rewound into another roll. Subsequently, various characters and figures such as product name and the like are printed on the resulting film. After printing, the film is folded and bonded along both edges by means of, for example, solvent adhesion, to give a tubular film (in tubing process). Meanwhile, there are cases where the sequence of the printing and the slitting processes is opposite. The tubular film thus obtained may be cut into tubular labels with a desired length, which is further converted into bags by bonding along an edge of openings thereof.
Subsequently, containers clothed in the labels or the bags above are passed, for example on a belt conveyer, through a shrinking tunnel such as the steam tunnel wherein heated steam is blown in for heat shrinkage and the hot-air tunnel wherein hot air is blown in, to give the final products (labeled containers) having the labels or the bags tightly attached thereto.
When variation in the heat shrinkage percentage of the labels, the bags, or others is large, there are occasionally produced defective labels and bags which do not exhibit proper heat shrinkage percentage in the heat shrinkage process, since the tunnel is controlled in a same heating condition during the production. As a result, large variation in the heat shrinkage properties leads to products inferior in appearance caused by insufficient shrinkage, shrinkage shading, crinkling, deformation in printed drawings, uneven shrinkage in the direction orthogonal to the maximum shrinkage direction, etc., which can not be shipped to market.
Usually, labels or bags for a specific product are produced from a single film roll, and thus large variation in heat shrinkage properties of a film in a film roll leads to a higher defective fraction in the heat shrinkage process. Further, when the solvent adhesion is employed in the tubing process above, a large variation in the solvent adhesiveness in the film feeding direction (longitudinal direction) of the film in a film roll often leads to defects in appearance of a tubular film produced from the film roll such as loss of planarity and corrugation of the solvent adhesion portion of the film due to swelling thereof by penetration of the solvent. When the variation in adhesiveness of the solvent adhesion of the tubular film is large, the portions bonded by the solvent adhesion in labels, bags or the like produced therefrom are occasionally separated during wrapping and shrinking thereof around containers in the wrapping and shrinking process, or during the storage of the final products after shrinkage. Further, since the tubular film become exposed under large pressure in the tube roll of the tubular film above, the portions of the tubular film where there are the defects of appearance such as corrugation described above often cause problems of a high incidence of blocking during storage in the roll.
The large variation in adhesiveness of the solvent adhesion also leads to problems in that when the tubular film is cut into labels, the cutting portions (opening) are fused by heat and consequently the resulting labels cannot be wrapped around containers, and in that the tubular film becomes difficult to be cut, causing defects in the cutting process.
The object of the present invention is to provide a heat-shrinkable polyester film roll and a process for producing the same, which can solve the various problems in the production processes described above, and thus decrease the incidence of defects in the products, during the processes of producing heat-shrinkable labels, bags or the like from a roll of a long film and of wrapping and shrinking the same around containers to produce labeled container products.